Dan Millis will never forget the day when he saw a deer family - a doe and two fawns - cautiously approach the US-Mexico border wall in the San Pedro Riparian National Conservation Area in Arizona. The mother deer kept looking at the wall and smelling it, trotting repeatedly and anxiously back and forth in search of what was once open land. Eventually she gave up attempting to cross, and warily led her fawns back the way that they had come.

"To me it was living proof that wildlife is being impacted in ways that we don't yet fully understand," Millis, director of the Sierra Club Grand Canyon Chapter Borderlands and Beyond Coal Campaigns, told Seeker. "A rancher who lives near that spot told me he has not seen the large deer herds on his land anymore, not since the wall went up."

Now there is to be even more wall, given President Trump's executive order on January 25 that calls for securing "the southern border of the United States through the immediate construction of a physical wall on the southern border, monitored and supported by adequate personnel so as to prevent illegal immigration, drug and human trafficking, and acts of terrorism."

The order clarifies that the "wall" will be a "contiguous, physical wall or other similarly secure contiguous, and impassable physical barrier."

President Trump and the Department of Homeland Security are waived from environmental regulations when building "security" infrastructure, per Section 102 of the Real ID Act passed in 2005 under the George W. Bush administration.

Congress would need to repeal or change the Section to allow for environmental regulations around the ordered additional "1000 miles of wall" called for by Trump. (The border between the U.S. and Mexico is 1,954 miles long, with walls already built along approximately 350 of those miles, and low-level vehicle barriers currently along 300 miles.)

Camera trap data from Arizona found that the wall did not prevent illegal immigrants from crossing into the United States, but wholly stopped wildlife movement.

Photo: Toad looking through the metal bars of part of the existing border wall. Credit: Anonymous. Provided to Seeker by Dan Millis

"Humans are good at building big, deep tunnels with electricity, and are good at building big ladders, catapults - not a joke - and using drones and more. Animals are not," Jesse Lasky, a Penn State biologist, told Seeker.

Even many U.S. Border Patrol agents, who have been on the front lines of the problem for years, support better surveillance over more construction. Some have advocated using aerial drones, for example, over building more wall, according to a Reuters report.

"I have yet to hear from anyone directly involved in interdiction and border security endorse building a sold concrete wall," Ecologist Tim Keitt at the University of Texas told Seeker.

"As a result, this pristine region - much of it on public lands set aside to protect rare and imperiled wildlife and sensitive habitat - is under siege," Bird said.

Environmental concerns, as outlined in the 2005 Border Ecological Symposium, include trampling of vegetation and other direct damage to wildlife and habitat, fragmentation of habitat and wildlife corridors, introduction of exotic species, air and water pollution, wildlife mortality and displacement, modifications of wildlife behavior in response to disturbances. The document also says the construction exerts added pressure on threatened and endangered species and presents obstacles to restoring these animals' habitats.

Impacts on WildlifeCountless species, including many that are rare and endangered, have already been impacted by the existing wall and will be threatened further by its expansion and increased human patrols, Bird and several others told Seeker.

According to the IUCN Red List of Endangered Species, the jaguar - the largest cat of the Americas - disappeared from the southwestern U.S. portion of its range by the mid-1900s, due to deforestation, hunting, trapping and other human activities. Anti-fur campaigns and other conservation efforts led to a slow comeback, "but fencing and road projects proposed by the Border Patrol threaten to cut off the cross-border corridors they use," Bird said.

The Trump order could devastate the Mexican gray wolf population in the region, he and others said. In 1998, the U.S. Fish and Wildlife Service released captive-bred Mexican gray wolves into Southeast Arizona. In 2011 Mexican gray wolves were also released into the Mexican state of Sonora about 30 miles from the U.S. border. Currently there are about 100 wolves north of the border and 35 south of the border.

"The wolves are at risk of inbreeding resulting in a genetic 'bottleneck,'" Bird said. "They need to freely cross the international border to improve their genetic pool."

He added that the Sonoran pronghorn antelope faces a similar fate, with a "serious possibility of extinction in the wild" due to the expected genetic bottleneck.

Then there is the cactus ferruginous pygmy owl, a tiny owl critical to desert ecosystems that rarely flies above five feet above the ground. The wall puts the owl at risk of local extinctions in Sonora as well as Arizona.

Still other species threatened by the past and planned border activities, according to conservationists, include the ocelot, jaguarundi, desert tortoise, black bear, cougar, desert mule deer, American bison, southwestern willow flycatcher, black-tailed prairie dog, yellow-billed cuckoo, and several plant and fish species.

Threats to Human HealthConcrete, one of the most-used construction materials in the world, releases pollutants during its production and is a known contributor to global greenhouse gases. According to a Bloomberg report, engineers at New York University and University College London recently concluded that a 1,000-mile wall would require 275 million cubic feet of concrete, and would release well over 2 million tons of carbon dioxide into the atmosphere.

"Beyond carbon dioxide, concrete production generates other pollutants that are highly toxic," Keitt said. "This would largely impact communities located near concrete plants along both sides of the border."

He believes the estimate of pollutants could be vastly understated, given that "no one really knows yet what it will entail to build a wall over mountain ranges in the region."

Another consequence of a longer wall is loss of land. Keitt pointed out that any border barrier cannot always be built directly on the border, and so could be placed far away from that location.

"Land at and behind a wall becomes inaccessible to owners and managers or may be taken using eminent domain laws or by other means," he explained. "It is a big deal in Texas because most of the border lands are privately owned."

Undoing 100 Years of ConservationConcern over protecting wildlife with ranges that go beyond the borders of any given country is nothing new. Clinton Epps, an associate professor of wildlife at Oregon State University, explained that "that since the U.S. entered the Migratory Bird Treaty Act of 1918, we've acknowledged the importance of protecting wildlife that moves across international borders."

President Woodrow Wilson, who led the nation through World War I, signed the Act before the war had even ended. It is still the primary U.S. legislation used to conserve migratory birds.

Aaron Flesch, a University of Arizona research scientist says that conservation efforts in the Mexico border region date to around that time too.

"If the government (now) wishes to mitigate the adverse effects of border development on wildlife and maintain the integrity of 100 years of conservation and management efforts in the region, they need to protect habitat, bolster the quality of that habitat, and take measures such as assisted migration or translocations that bolster connectivity for the species most likely to be affected," he told Seeker.

Alternatives to the Solid WallDefenders of Wildlife argues that a wide range of high-tech monitoring devices are now available that would provide security while minimizing the impact on wildlife. They include prioritizing usage of "virtual" high-tech fencing options - such as unmanned aerial vehicles, motion-sensors, laser barriers and infrared cameras - and using wildlife-friendly vehicle barriers in conjunction with virtual fencing in areas where hard infrastructure is necessary and appropriate.

Photo: Dead bird in the bars of the existing border wall. Credit: Anonymous. Photo provided to Seeker by Dan Millis

He added that fences are only appropriate directly adjacent to urban areas and should not be used in wildlife corridors or other ecologically sensitive areas.

Support for WildlifeA recent poll conducted by the Center for American Progress found that 91 percent of voters said it was either a very important or a fairly important goal of the federal government to protect and maintain national parks, public lands and natural places, and to protect those natural places for future generations. Eighty-one percent of voters also said it was either very important or fairly important to save at-risk wildlife from extinction.

"Protecting species from extinction receives strong bipartisan support," Bird said. "It is important that the public knows how much damage and destruction a contiguous, impenetrable border wall will have for wildlife and their habitat."

Defenders of Wildlife and other environmental organizations are now speaking with members of Congress about the projected impacts that an impenetrable border wall will have on animals, plants and the overall landscape. Congress will likely need to approve funding for any construction in an appropriations bill, which will possibly be a supplemental appropriations bill.

Millis, who spoke with Seeker from the borderlands region, said that the affected lands "are among the most majestic, large and connected natural areas in the world."

"Most of the border," he said, "is made of water such as the Rio Grande and the Colorado River. When you think of the border, don't think of a wall; think of water and wildlife."

Top photo: Border wall between United States and Mexico prevents deer from crossing. Credit: Anonymous. Provided to Seeker by Dan Millis WATCH:Are Borders Crimes Against Geography?